The present invention relates to a cordless key telephone system.
Cordless key telephone systems have been in use today replacing wired key telephone systems as they considerably reduce the amount of cables and wires laid on office floors the arrangement of which must be altered to meet current needs of working environment. The system includes a main controller which is coupled to the public or private switched telephone network via subscriber loops to receive incoming calls and originate outgoing calls. The floor space which the system covers is divided into several service areas. One or more access units are located in each service area and coupled to the main controller to exchange control signals with it. In response to an incoming call, the main controller selects one access unit for each service area and sends an alert signal to it. Each selected access unit then broadcasts a recurrent sequence of alert signals respectively containing the identifiers of all standby cordless stations to elicit a response therefrom. Each standby cordless station responds to the alert signal addressed to it by returning an acknowledgment signal containing in it the own station identifier as well as the system identifier. Each selected access unit waits until it receives the acknowledgment signals from all standby cordless stations and communicates this fact to the main controller. The main controller waits for this communication from each selected access unit to return a proceed-to-assign signal to allow it to broadcast a channel assignment signal to switch the standby cordless stations to an assigned speech channel on which ringing signals will then be transmitted. If this communication is not received within a prescribed timeout period, the main controller automatically sends the proceed-to-assign signal to each selected access unit. When one of the cordless station goes off hook, this fact is signaled by the associated access unit to the main controller, whereupon it establishes a connection between the network and the access unit from which the off-hook condition is signaled.
Since the cordless stations may roam across the boundaries between the service areas of the system and since the access units have no way of knowing this fact, the timeout period for receiving acknowledgments from all stations often expires, resulting in a delayed action for answering an incoming call.
In addition, it is the usual practice to include a battery savings circuit in the cordless stations. Upon completion of an incoming call to a certain standby cordless station, the other standby cordless stations are automatically switched to the battery savings mode. If another incoming call is received immediately following the previous call, however, all other cordless stations are in their battery savings mode and there is an inevitable delay in alerting the stations until their power supplies are turned on again.